Hyper-V XP pro guest: tweaking number of processors

I have a XP pro (32 bit) Hyper-V guest running on Windows server 2012 R2 std
I ran a test application with various quantities of virtual processors:
1    Processor application ran slow.
4    Processors application ran faster.
32 Processors application ran very slow.
Is there a better way to find the optimum setting than trial & error?
Thx,
SA_AdminAsked:
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Cliff GaliherCommented:
Not really, and this isn't necessarily because of Hyper-V.  Keep in mind that XP released before multi-core processors were common, so multi-threading is not its strongest ability.  If you assign more processors than you have physical cores, you are now introducing some overhead to Hyper-V that'll hurt you as well. So I think you hit two extremes. One with too little, and the other you just overwhelmed Hyper-V.  On modern hardware, 1 processor is usually fine for XP though, so I suspect you have some unrelated issues going on as well.  Hyper-V has a ton of performance counters you can use to get insight into what is actually happening on the machine.
Lee W, MVPTechnology and Business Process AdvisorCommented:
First, XP Pro limits to 2 CPUs so any more than that are ignored by XP.

Second, the problem could be CPU overprovisioning.  I'm not CERTAIN that Hyper-V works the same as VMWare, but the more CPUs your assigning, the hypervisor must WAIT for ALL the CPUs to be idle to give them to the VM.  If your host server runs 2x8 core 16 thread processors, then ALL must have a moment of "idleness" to allocate ALL to the VM.  

GENERALLY, you want to minimize the number of CPUs you assign.  I personally like giving at least 2 to a VM - I find that's generally a sweet-spot for most non-CPU intensive systems.

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Lee W, MVPTechnology and Business Process AdvisorCommented:
Philip ElderTechnical Architect - HA/Compute/StorageCommented:
Hyper-V limits XP Pro to 2 virtual CPUs (vCPUs).

That's going to be your maximum.

As Lee mentioned, your host's workloads could also be a factor.
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